PROJECT SUMMARY/ABSTRACT Glioblastoma (GBM) is a highly aggressive malignancy with very high morbidity and mortality, due to lack of effective therapies. The overall goal of this proposal is to identify novel cellular targets in GBM cells that could lead to new therapeutic approaches. We have found that a member of the Schlafen (SLFN) gene family, SLFN5, is significantly overexpressed in GBM as compared to normal brain, and that high levels of SLFN5 expression correlate with poor survival among GBM patients. Our data indicates that SLFN5 promotes GBM growth by repressing IFN signaling and IFN stimulated gene (ISG) expression via an interaction with the transcriptional activator STAT1. This highly novel finding forms the basis of the current proposal. Aim 1 will identify elements of SLFN5-STAT1 complexes, define upstream regulatory signals required for the formation of such complexes, and determine relationships between elements of these complexes and SLFN5-associated transcriptional repression. The functions of different SLFN5 structural motifs and their importance to the suppression of IFN-responses will be examined. Studies using primary samples from GBM patients will be also employed for such studies. Aim 2 will define the effects of SLFN5 expression in vivo using three distinct orthotopic engraftment models and GBM cell pairs with and without SLFN5 expression: i) conventional xenograft models using athymic mice for examining the effects of SLFN5 on tumor establishment and growth; ii) humanized orthotopic PDX models to examine SLFN5 effects for human-on-human immune response against tumor, as well as to examine tumor response to immune checkpoint therapy; and iii) same a ii but using mouse GBM syngeneic models in which the host animals have a fully functional immune system. Altogether, the results of this work will provide important information on the mechanisms by which SLFN5 expression suppresses IFN-responses and promotes GBM growth. The successful performance of this work should facilitate development of highly novel approaches for the treatment of GBM using SLFN5 as a target.